Using this little gem (thanks Stephen), here's a function which appears to calculate age in years, months, days, hours, minutes, seconds and milliseconds. Not only that: if you use DATEADD to add all of these units to the DOB, the result is the "date as of":
ALTER FUNCTION [dbo].[il_ElapsedTime]
/*
Calculate the elapsed time between two datetimes
as year, month, day, hour, minute, second, millisecond
such that adding these values using DATEADD to the earlier
value will yield the later value.
This also calculates AGE.
*/
   (@Then DATETIME, @Now DATETIME)
RETURNS TABLE WITH SCHEMABINDING AS
RETURN
SELECT
   ds.Years,
   mo.Months,
   ds.[Days],
   r.[Hours],
   r.[Minutes],
   r.Seconds,
   r.Milliseconds
FROM (
 SELECT
  [Days] = CASE
   WHEN DAY(@Now) >= DAY(@Then) THEN DAY(@Now) - DAY(@Then)
   ELSE DAY(@Now) + (DATEDIFF(DAY,@Then,EOMONTH(@Then)))
   END,
  [Years] = (0 + CONVERT(CHAR(8),@Now,112) - CONVERT(CHAR(8),@Then,112)) / 10000
) ds
CROSS APPLY (SELECT [Months] = DATEDIFF(month,DATEADD(YEAR,[Years],DATEADD(day,[Days],@Then)),@Now)) mo
CROSS APPLY (
   SELECT
      [Hours] = DATEPART(HOUR,@Now-@Then),
      [Minutes] = DATEPART(MINUTE,@Now-@Then),
      [Seconds] = DATEPART(SECOND,@Now-@Then),
      [Milliseconds] = DATEDIFF(MILLISECOND,DATEADD(SECOND,(CASE WHEN DATEPART(MILLISECOND,@Then) > DATEPART(MILLISECOND,@Now) THEN -1 ELSE 0 END),DATEADD(MILLISECOND,DATEPART(MILLISECOND,@Then),DATEADD(MILLISECOND,0-DATEPART(MILLISECOND,@Now),@Now))),@Now)
) rWHERE @Now > @Then

Here's a test harness too:
SET NOCOUNT ON
DECLARE @Today DATETIME = GETDATE()-13
IF OBJECT_ID('TempDB..#DOBs') IS NULL BEGIN
 SELECT DOB = DATEADD(DAY,1-ROW_NUMBER() OVER(ORDER BY (SELECT NULL)),CAST(@Today AS DATETIME))
 INTO #DOBs
 FROM
  (VALUES (0),(0),(0),(0),(0),(0),(0),(0),(0),(0)) d1 (n),
  (VALUES (0),(0),(0),(0),(0),(0),(0),(0),(0),(0)) d2 (n),
  (VALUES (0),(0),(0),(0),(0),(0),(0),(0),(0),(0)) d3 (n),
  (VALUES (0),(0),(0),(0),(0),(0),(0),(0),(0),(0)) d4 (n),
  (VALUES (0),(0),(0),(0),(0)) d5 (n)
CREATE UNIQUE CLUSTERED INDEX ucx_Stuff ON #DOBs (DOB)
END

SELECT
 d.DOB,
 Today = @Today,
 x1.*,
 NewDate =
  DATEADD(YEAR,x1.Years,
  DATEADD(MONTH,x1.Months,
  DATEADD(DAY,x1.[Days],
  DATEADD(HOUR,x1.[Hours],
  DATEADD(MINUTE,x1.[Minutes],
  DATEADD(SECOND,x1.[Seconds],
  DATEADD(MILLISECOND,x1.[Milliseconds],d.DOB)))))))
FROM #DOBs d
CROSS APPLY [dbo].[il_ElapsedTime] (d.DOB, @Today) x1
-- Without the NewDate calculation and with rows blackholed
-- into variables; 50,000 rows in about half a second (SET STATISTICS TIME)

Edit: Sorry, but the code display seems to be completely haywire today. If I enter EDIT mode, the code tags look fine.